1. Field of the Invention
This invention relates to improvements in a triplet objective lens, and in particular to a behind stop modified Tessar type objective lens of compact three-group four-lens construction in which the angle of view amounts to 60.degree. or greater and the brightness amounts to F3.5.
2. Related Background Art
A so-called Tessar type lens which is an objective lens comprising three components, i.e., a diverging biconcave lens and two converging portions with the diverging biconcave lens interposed therebetween has the features that its full length is relatively short and that the fluctuations of aberrations by the object distance are relatively small, and is known as a compact objective lens optical system. Particularly, a lens of the behind stop type in which a stop is provided rearwardly (on the image side) of the lens system permits distance adjustment to be accomplished by fixing the stop on the optic axis and axially moving only the lens and therefore is most suitable for the distance adjusting mechanism of a compact auto focus camera, and has often been put into practical use. In the ordinary Tessar type lens, a third component has a cemented surface convex relative to the object side, but many modified Tessar type lenses in which the third component has a cemented surface concave relative to the object side are also known. In such modified Tessar type, the sagittal plane is somewhat flat and the full length of the lens can be balanced to a slightly short length.
Generally, however, in the case of an objective lens provided with a behind stop, the light ray of an oblique light flux which passes through the marginal portion of a first lens passes through a location near the center of the stop and therefore, a decrease in the marginal light is liable to become conspicuous, and this leads to the disadvantage that it is difficult to correct the deficiency of the quantity of marginal light even if the variable stop is stopped down. In a Tessar type lens comprised of three components, it is possible to reduce the decrease in the marginal light considerably by making the effective diameter of the first lens large, but occurrence of coma and chromatic coma is conspicuous. Particularly, in the conventional modified Tessar type lens in which a third component has a cemented surface concave relative to the object side, occurrence of chromatic coma has been remarkable.